In wireless communication systems, antenna arrays are used at devices on one or both ends of a communication link with multipath fading and interference, and to increase system capacity by supporting multiple co-channel users and/or higher data rate transmissions through the use of beamforming and/or multiple-input multiple-output (MIMO) techniques, which are emerging technologies used in newer wireless communication systems.
A base station equipped with an antenna array can improve downlink MIMO transmission performance by using uplink channel information from transmissions received from a mobile station. In general, the base station can only estimate, i.e., not determine with certainty, the downlink (DL) channel spatial characteristics from the uplink (UL) channel characteristics using spatial decomposition of UL transmissions because the spatial and frequency dependent aspects of the UL and DL channels are different. Current techniques for estimating DL channel information rely on narrow band direction of arrival (DOA) analysis in which the UL sub-channels at each DOA are analyzed as a single path or a flat fading channel to compute a direction of departure (DOD) for DL transmissions. These techniques have limitations when applied in a wideband communications system.